Split gate flash memory cells are well known in the art. One such well known example is disclosed in U.S. Pat. No. 5,029,130 whose disclosure is incorporated herein by reference in its entirety.
As shown in FIG. 1 of U.S. Pat. No. 5,029,130, a split gate flash memory cell 10 comprises a semiconductor substrate 12, with a first conductivity type, such as P. A first region 14 of a second conductivity type, such as N is along the surface of the substrate 12. Spaced apart from the first region 14 is a second region 16 also of the second conductivity type N. Between the first region 14 and the second region 16 is a channel region 18. A floating gate 22 is positioned over a portion of the channel region 18 and over the first region 14 and is capacitively coupled to the first region 14. A control gate 29 has two portions: a first portion 30 is laterally adjacent to the floating gate 22 and is disposed over another portion of the channel region 18, and has little or no overlap with the second region 16. The control gate 29 has a second portion 28 which is connected to the first portion 30 and is disposed over the floating gate 22 (only extends part way over floating gate 22 to reduce capacitive coupling therebetween—i.e. only a weak capacitive coupling therebetween).
During the read operation, a zero or near zero voltage is applied to the second region 16, with a first positive voltage applied to the first region 14. A second positive voltage is applied to the control gate 29 turning on the portion of the channel region 18 underneath the control gate 29. In the event the floating gate is programmed, i.e. has stored electron charges, then the positive voltage on the control gate 29 and the positive voltage on the first region 14 are not sufficient to turn on the portion of the channel region 18 underneath the floating gate 22. The low or zero current through the channel region is detected as a first state (e.g. a 1). However, if the floating gate 22 is not programmed (i.e. it lacks stored electron charges), then the positive voltage on the control gate 29 and the positive voltage on the first region 14 are sufficient to cause the floating gate 22 to be capacitively coupled to a more positive voltage, which turns on the portion of the channel region 18 underneath the floating gate 22, thereby turning on the entire channel region 18. The current through the channel region is detected as a second state (e.g. a 0). However, as cell size has shrunk, and dimensions reduced, and the voltages lowered, increasingly it becomes difficult to turn the floating gate 22 on when it is erased. After program-erase cycling, the potential of the floating gate 22 of the erased cell becomes even lower because of electron charge trapping in the tunnel oxide (separating the floating gate 22 and control gate 29) and thus the corresponding decrease of tunneling efficiency. Thus, it is desired to have an additional gate to assist during the read operation, and still be process compatible with the above-described cell.
U.S. Pat. Nos. 6,855,980 and 7,315,056 each disclose a flash memory cell with a floating gate, a control gate to one side thereof and an assist gate over the first region to another side of the floating gate. However, these two patents (whose disclosures are incorporated herein by reference in their entirety) do not disclose the method of using that cell to enhance read and erase operations.
Accordingly, it is one object of the present invention to provide a method of operating a cell during read and erase operations.